The present application claims priority under 35 U.S.C. xc2xa7119 of Austrian Patent Application No. 939/2000, filed on May 29, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a process for hardening a rail or part thereof (e.g., the rail head) by forced cooling and a device for carrying out a corresponding process.
2. Discussion of Background Information
For the growing railway traffic with its increasing axle loads rails should, on the one hand, have high wear resistance in the area that comes into contact with the train wheels and, on the other hand, have high resistance to fracture in view of the high bending load acting on the rail.
Thermally tempering the (steel) material or changing the structure from an austenitic structure into a microstructure that is stable at room temperature during at least temporary force-cooling is known to harden the rail and/or the rail head (EP-358362 A1, AT 402941 B, EP186373 B, WO 94/02652).
Cooling can be effected by subjecting at least a part of the rail surface to coolant, a so-called splash cooling or spray cooling, or by at least partially submerging the rail into a coolant bath, wherein the advantageous use of the rolling heat is known in the art.
Depending on the cooling method used, pass-through processing devices (AT 323224 B, EP 186373 B1), cooling bed transport devices (DE 4237991 A1) and submerging devices (DE 4003363 C1, AT 402941 B) are known for forced cooling of the rail or parts thereof.
When alloys with an appropriate chemical composition are used, rails with increased hardness and wear resistance in the area of the surface of the rail head and with sufficient resistance to fracture can be produced by using hardening processes in the appropriate devices.
A possible lack of homogeneity of the structural distribution over the cross-section as a function of the length of the rail must be considered a great disadvantage of the known hardening processes and cooling devices for rails. In other words, if the portion of the surface area of the relevant tempering structure and/or the position of the structural constituents in the rail cross-section is uneven over the length of the rail this has an increasingly detrimental effect on the quality of the rail. Even if process parameters are maintained precisely and cooling devices are controlled precisely, unexpected differences in the quality of the rail can occur, resulting in some individual rails that do not meet the quality requirements of an extremely sophisticated quality control.
The present invention is directed to overcoming the above problems by providing a process of the above mentioned type which results in a constant structural distribution over length across the rail cross-section and a high rail quality.
The present invention furthermore is directed to providing a device for hardening a rail or parts thereof through which the local cooling intensity of the surface regions of the cross-section can be kept constant over the length of the rail.
One aspect of the present invention is a process for hardening a rail or part thereof by transforming it from an austenitic structure into a different microstructure that is stable at room temperature, the process comprising aligning, horizontally positioning, and fixing in axial alignment to secure against bending, the rail in its austenitic state; and while keeping the rail fixed in axial alignment and secured against bending, force-cooling the rail or part thereof to allow the austenitic structure to be transformed into said different microstructure.
In one embodiment of the above process the rail is force-cooled from a first temperature above its Ac3-point to a second temperature which is below said Ac3-point. In another embodiment of the process only the rail head is force-cooled.
It may also be advantageous if alignment, positioning and fixing in axial alignment are conducted immediately after the last finish rolling pass.
In the above process the rail may, for example, be fixed in a standing position, with the rail head pointing (straight) upward, or it may be fixed in a hanging position, with the rail head pointing (straight) downward.
Force-cooling of the rail or part thereof may be accomplished by spray cooling, for example by spray cooling while using equally high cooling intensities in the surface regions of the cross-section symmetrically to the height axis of the rail viewed in longitudinal direction.
In another embodiment of the process of the invention the rail or part thereof may be force-cooled by immersion thereof into a cooling liquid.
In a further embodiment the rail or part thereof may force-cooled intermittently with respect to at least one of time and location with regard to a surface region of the cross-section.
After the force-cooling the rail may be released and kept at an elevated temperature, and/or it may be left to cool in ambient air.
In many cases the length of the rail will be at least about 50 m, e.g., at least about 90 m, and frequently the rail or part thereof will be force-cooled over approximately its entire length.
In another aspect the rail is fixed in axial alignment by means of fixing elements arranged in longitudinal direction of the rail at a distance to each other of not more than about 1 m, e.g., not more than about 0.5 m. These fixing elements may be designed to keep the rail foot in a fixed position.
The Ac3-point referred to above is the temperature of iron or an alloy thereof at which upon heating a purely austenitic (xcex3) microstructure is present. For more details as regards the definition of the Ac3-point (and microstructures that are stable at room temperature) reference may, for example, be made to F. Rapatz, xe2x80x9cDie Edelstxc3xa4hlexe2x80x9d, Springer-Verlag Berlin, Germany, 1962, pp. 2-25 (see particularly pages 3 and 12), the disclosure of which is expressly incorporated by reference herein in its entirety.
The advantages achieved through the process of the invention include that alignment occurs in the austenitic structural state and heat is then removed from the rail surface at an increased rate, while it is fixed (clamped) in axial alignment. During the intensive cooling of the rail cross-section or parts thereof the rail remains fixed in axially straight position which helps to keep the specific cooling intensity constant in the axial direction. Extensive research shows that, if even a slight bending of the rail occurs during intensive cooling of the rail or at least parts thereof, the local cooling rate curve in the surface region can change. This has a major effect on the formation of the structure during the change from the austenitic state of the alloy. In a thermal tempering of the rail an axially aligned (flush) horizontal mounting according to the invention secures a constant profile of material properties over the cross-section and over the length of the rail.
A particularly economical embodiment of the process of the present invention is obtainable when aligning (straightening), positioning and fixing in axial alignment of the rail are performed immediately after the last finish rolling pass with utilization of the rolling heat.
For a specific site technology, but also for a desired microstructural distribution over the cross-section of the rail, it may also be advantageous to fix (clamp) the rail in a standing position, with the rail head pointing straight upward. Here, it is advantageous to remove the heat from the rail by spray cooling using equally high cooling intensities in the surface regions of the cross-section symmetrically to the vertical axis of the rail viewed in the longitudinal direction.
In order to improve manufacturing reliability of rails with the desired property profile and to achieve special wear resistance of the surface in contact with the train wheels, it may be advantageous if the rails are mounted hanging with the rail head pointing vertically downward. Another reason why such positioning may prove beneficial is that this way heat may be removed by submerging the rail or only a part thereof (particularly the rail head) into a cooling liquid.
For controlled cooling to a desired temperature with a high cooling intensity of the cooling medium and an interruption of the forced cooling it may be advantageous in terms of transformation kinetics if the cooling of the rail is performed intermittently with respect to time and/or location with regard to a surface region of the cross-section. Here, it may also be beneficial to release (e.g., unclamp) the rail, after the force-cooling, and to keep it at an elevated temperature and/or to allow it to cool in air at ambient temperature.
Use of the present process in which a cooling or hardening or a thermal tempering of the rail occurs over the full length thereof has proven to be particularly beneficial for high uniformity and high quality as well as the attainment of optimal service properties.
Another aspect of the present invention is a device for hardening a rail or part thereof by force-cooling. The device comprises a support for supporting the rail; a cooling device for force-cooling the rail or part thereof; and at least two fixing devices for axially aligning and securing the rail against bending during the force-cooling.
The support advantageously comprises a support structure having a high resistance to bending. This support structure may comprise a welded structure having the fixing devices horizontally arranged thereon.
In one embodiment of the present device the fixing devices thereof comprise at least one element selected from positioning elements, releasable clamping elements, depressing elements and combinations thereof. The depressing elements may, e.g., be in the form of hold down weights.
In many cases the device of the present invention may comprise at least three fixing devices, e.g., at least about 50 or at least about 100 fixing devices. The fixing devices may be arranged in longitudinal direction of the support structure at a distance of not more than about 1 m from each other, e.g., not more than about 0.5 m from each other. The fixing devices can be aligned horizontally, and at least one of them may comprise a positioning element for contacting an upper surface of the rail foot and a depressing element for contacting a lower surface of the rail foot. In another embodiment at least one fixing device may comprise a pair of releasable clamping elements having horizontally arranged alignment surfaces for contacting the rail.
One or more elements of the fixing devices that come into contact with the rail may have a shape that reduces the contact area with the rail. For example, they may be wedge-shaped.
In another embodiment of the device of the present invention the cooling device comprises a spray track with at least one of air and water. The cooling device may also comprise an immersion pool with a cooling liquid.
In another aspect of the present device, the support and the cooling device are movable relative to one another in the direction of the vertical of the rail in the cross-section. Furthermore, the support may be connected with a immersion pool having fixing devices comprising horizontally aligned positioning elements.
The support of the present device is desirably of about the same length as the rail to be supported.
An advantage resulting from a support structure that is resistant to bending and torsion is an axially aligned fixing or clamping of the rail even if during intensive cooling bending forces are generated due to different weight distribution and/or different cooling intensity over the cross-section. Just as important are the advantages of an axially aligned fixing in relation to the application of cooling medium to the surface, or wetting of the surface by cooling medium, because this way the exact alignment in desired areas of the rail is made possible with a high uniformity over the length of the rail. This way desired cooling rates, and thus desired microstructures, can be accomplished with high accuracy for predetermined cross-sectional zones.
Due to the site technology, but also with regard to use, it may be beneficial if the support structure is realized as a welded construction, on which at least three fixing devices are mounted, preferably at a distance of not more than about 0.5 m from each other over the longitudinal extent, such that they can be aligned horizontally.
A simple and reliable device is realized when releasable fixing elements take the form of depressing (holding down) elements for rails in contact with positioning elements.
It is also possible, advantageously, to design releasable fixing elements with alignment surfaces for the positioning of the rail horizontally in the axial direction.
In order to achieve a microstructure that is as uniform as possible over the length of the rail or to avoid any substantial influence on the local cooling intensity, it may be advantageous for the fixing elements, e.g., positioning elements and clamping elements, to have a reduced area of contact with the rail, for instance, to be wedge-shaped. Thus so-called xe2x80x9csoft spotsxe2x80x9d can be avoided on the rail.
With a careful nozzle positioning and/or the utilization of water essentially free of suspended fine particles, it can be advantageous if the cooling device for the rail is designed as an air and/or water spray track with even cooling intensity over the longitudinal stretch of the rail.
If, on the other hand, the cooling device is designed as an immersion pool with a cooling liquid, the cooling intensity that acts on the immersed areas of the rail can easily be adjusted by adding synthetic substances.
Further, if the device is designed so that the rail support structure and the cooling device can be moved relative to one another in the direction of the rail vertical in cross-section, cooling cycles and/or the cooling of rail parts can be performed in a particularly efficient manner.
A particularly simple device can be realized or constructed as a retrofit, if the support structure is connected with an immersion pool comprising horizontally aligned fixing devices and the rail can be caused to contact positioning elements by means of mounting elements, for instance depressing elements. In this case the mounting elements can be designed in a particularly simple manner in the form of holding down weights, being arranged at least in the distal areas of the rail.
For an isothermal heat treatment, so to speak, it is advantageous if the device can be used for discontinuous hardening of rails or cross-sectional parts thereof.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.